A device specifically designed to be inserted into an optical transmission line and coupled to one or more semiconductor type photosensitive elements all of which are contained in an opaque enclosure. Light input to the device by an external source acts upon the light sensor(s) to produce a corresponding output or change in some electrical quantity or variation in some electrical property. For items activated by an internal light source by application of an electrical input and having an electrical output, see COUPLER, OPTOELECTRONIC.